Add new command: git trac old.

[roed314]

As discussed on this sage-devel thread, it is frustrating to checkout an old branch and have it touch most of the files in Sage's source tree, since this essentially forces a total rebuild.

This ticket implements a simple fix: add a new command that instead checks out develop first, then pulls in changes from the old branch.

[novoselt]

A similar discussion and as it seems to me a different solution here: https://trac.sagemath.org/ticket/22451

I did not understand from the documentation what exactly this old command does - merge into develop so that develop now contains those old things??? What we really want is, I think:

• start on fresh develop or perhaps some other recent branch which is close to develop
• git checkout old_branch
• git merge develop
• end up in a state where make will not recompile everything.

git trac old old_branch does not seem like a natural choice of words, perhaps something like git trac merge-develop-into old_branch would make more sense?